High temperature, solid oxide electrolyte fuel cell generator apparatus and fuel cell configurations are well known, and taught, for example, in U.S. Pat. No. 4,395,468 (Isenberg). There, feed fuel, either H2+CO, or previously reformed natural gas, is fed into the apparatus at one end and flows parallel to exterior fuel electrode surfaces of elongated tubular fuel cells. Spent fuel is combusted with spent oxidant in a separate combustion chamber and then exits the apparatus. In this patent a power generation chamber extends between an end wall of the generator housing and a porous, stiff positioning/separator board on the other side of which is a combustion chamber containing open ended fuel cells into which air/oxidant feed tubes are passed. The combustion chamber pre-heats incoming air/oxidant passing through the air/oxidant feed tubes and into the interior of tubular closed end fuel cells. Other generator designs utilize an open end fuel cell structure, for example, U.S. Pat. No. 5,200,279 (Draper et al.), or flattened rather than tubular fuel cells, for example, U.S. Pat. No. 4,888,254 (Reichner).
In later SOFC embodiments, some spent fuel from the recirculation chamber between the combustion chamber and the power generation chamber, as shown in FIG. 2 of U.S. Pat. No. 4,983,471 (Reichner and Dollard) was recirculated either directly from the recirculation chamber or from openings in the power generation chamber. A similar arrangement is shown in FIG. 4 of U.S. Pat. No. 5,573,867 (Zafred et al.), where some spent fuel captured in a spent fuel recirculation chamber is recirculated to an ejector to mix with fresh feed fuel. The fuel cell positioning board, between a combustion chamber and a spent fuel recirculation chamber is more clearly shown in FIG. 2 of U.S. Pat. No. 6,656,623 B2 (Holmes et al). In some instances, the open end of the fuel cell, located in the combustion chamber, was vulnerable to reduction by spent fuel. Originally, this was solved by Zafred et al., in U.S. Pat. No. 6,221,522 B1, by utilizing open fuel cell end sleeves, but this added to costs and did not completely solve the problem.
Originally, upper and lower positioning boards were designed to function as loose separators between the power generation fuel cell stack region, the spent fuel recirculation plenum/chamber and the combustion chamber, and also as open-end cell positioning boards. Typical board material was a low density, vacuum-formed fibrous ceramic board, which was subsequently machined to obtain loose clearance holes for the fuel cells and flow holes through the ligament between adjacent cells.
However, inadequate sealing tended to cause some fuel and temperature mal-distribution. Also, differential axial thermal expansion between the cells and the positioning boards, could lead to cells “grabbing” the boards during generator shutdowns and potentially breaking them. Additionally, the use of initially stiff positioning boards required 100% location mapping using a very expensive laser mapping technique which generates a series of XY coordinates for each cell position. These data are inputted into a milling machine which machines the final custom hole pattern to precisely match the irregular cell pattern configuration.
What is needed are positioning boards which are both flexurally strong yet soft and compliant and also cost-effective. Furthermore, they should have specific permeation characteristics to allow only a certain fraction of spent fuel to flow through to the combuster chamber while allowing a greater fraction to recirculate through the stack. They also should be initially flexible during insertion into the generator and should also somehow have the capability of protecting the open end of the fuel cell in the combustion chamber from localized burning of hydrogen during transient operating conditions where normal temperatures of 1100° C. can rise for a short time to as high as 1400° C. It is a main object of this invention to provide novel positioning boards, especially those interfacing with the combustion chamber, where the boards meet strong yet compliant requirements and somehow are effective to protect fuel cell open ends during short transient conditions.